1. Field of the Invention
This invention relates to conveyor systems, and more particularly, to shiftable conveyor systems which can be laterally displaced substantial distances.
2. Description of the Prior Art
In strip mining operations, overburden is first removed from an underlying deposit of the desired material. For discussion purposes, it will be assumed that the desired material is coal. After overburden removal, the coal is mined in tiers which are joined by an advancing vertical wall of coal. The mined coal must be conveyed out of the mining site to an accessible location from which the coal can be transported to the point of sale or to a processing plant.
Standard strip mining procedure involves the use of an electrically powered drag line or shovel which removes large volumes of coal from the vertically oriented coal face. The drag line deposits the mined coal onto a system of conveyor belts which transport the coal to storage or transportation sites adjacent to the strip mine.
As the mining operation progresses, the drag line removes substantial portions of the coal face requiring advancement of the mining equipment in the direction of the receding coal face. Typically, after a one hundred foot strip has been removed from the coal face, the conveyor system must be repositioned to be adjacent the mining equipment and drag line. This relocation of the conveyor system usually takes eight hours and requires that all mining operations be temporarily suspended.
The shift rates for conveyor systems of the type currently in use in strip mining operations are normally stated in terms of square feet per hour. This unit of measure defines the area covered during lateral movement of the conveyor during a one-hour period of continuous transposition. The maximum shift rate typically characterizing prior art conveyor systems is on the order of twenty-five thousand square feet per hour. For a typical one-half mile long conveyor system, an eighty foot lateral shift will cover 211,200 square feet. Since the fastest prior art shiftable conveyor systems can shift twenty-five thousand square feet per hour, a shift of 211,200 square feet would take approximately eight hours and twenty minutes. During this period of time the extraordinarily expensive drag line must be shut down since there is no means for conveying the coal which it removes from the coal face to a point outside the mine. All equipment for transporting the coal from the strip mine to the point of sale is unable to function unless the drag line and its associated conveyor equipment are duplicated at some other point in the mine. Additionally, during the time required to shift the conveyor system into a new operating position, salaried employees are underutilized. Since the times at which the conveyor system must be shifted depend on the rate at which the coal face is mined, operating personnel often cannot be scheduled to be off duty during these time consuming conveyor shifting operations.
While it is possible to shift prior art conveyor systems more frequently for distances of up to ten to twenty feet at a time, the cumulative down time for a given amount of lateral displacement remains the same because the shifting rate of the conveyor system cannot be increased by accomplishing shifts in smaller increments.
Since various elements of a conveyor system are deformed during a shift, prior art shiftable conveyor systems must be realigned following each shift. For this reason, a shorter shift distance is less practical since each shift requires a separate time consuming realignment step.
An additional negative aspect of prior art shiftable conveyor system relates to profits from coal sales lost as a result of the conveyor down time. A down time of approximately eight hours results in a loss of approximately 24,000 tons of coal production.
By reducing the down time of a shiftable conveyor system, the overhead costs associated with strip mining are reduced and the profits resulting from the sale of coal are increased: overhead utilization is increased and larger amounts of coal are mined during a fixed time interval.
Various systems have been suggested in the past to facilitate the lateral movement of a conveyor. U.S. Pat. No. 3,612,257 (Goergen) discloses a shiftable conveyor system which permits the belt of the conveyor system to be laterally shifted up to several feet. A plurality of wheels are provided to allow the conveyor system to be longitudinally displaced substantial distances. The ability of this conveyor system to be laterally displaced apparently is directed toward providing conveyor belt alignment upon completion of a large longitudinal displacement of the conveyor system by towing.
U.S. Pat. No. 3,796,296 (Bakker) and U.S. Pat. No. 2,722,306 (Cartlidge) disclose articulated segments of a conveyor system for allowing the conveyor system to operate in other than a linear manner. No means for laterally displacing the conveyor system is disclosed.
German Pat. No. 507,904 discloses a conveyor system having laterally shiftable elements to provide for non-linear operation of the conveyor system. This conveyor system is not designed to be laterally displaced along its entire length for substantial distances.